Polishing composition and polishing method

ABSTRACT

A polishing composition includes colloidal silica. The colloidal silica has such a particle diameter as to satisfy the relationship of the inequality: D SA ≦D N4 , between an average primary particle diameter D SA  of the colloidal silica calculated on the basis of a BET method and an average secondary particle diameter D N4  of the colloidal silica measured by a laser diffraction method. The colloidal silica has the average secondary particle diameter D N4  of 30 nm or smaller. The polishing composition can inhibit a polishing rate from remarkably decreasing due to clogging in a polishing pad.

BACKGROUND OF THE INVENTION

The present invention relates to a polishing composition used, for example, for polishing a substrate with the use of a polishing pad, and to a polishing method using such a polishing composition.

In a process for polishing a substrate such as a silicon wafer, generally, the improvement of surface quality is considered as a major problem with a requirement for a higher performance and higher packing density of semiconductor devices, and also is the improvement of manufacturing efficiency in order to cope with grow in demand in recent years. For the purpose of solving the problems, Japanese Laid-Open Patent Publication No. 5-154760 discloses a polishing composition having an improved polishing rate (stock removal rate). The polishing composition includes colloidal silica sol or silica gel and piperazine, and the weight of piperazine in the polishing composition is 10 to 80% of the weight of SiO₂ in the colloidal silica sol or the silica gel in the polishing composition. The surface of a silicon wafer is chemically and mechanically mirror-finished by mutual rotation between the silicon wafer and a polishing pad which is in the state of pushing the silicon wafer fixed on a ceramic block, while the polishing composition is supplied to the polishing pad.

While the polishing pad is repeatedly used for polishing a substrate, it causes clogging therein, and a polished amount per unit time, or equivalently, a polishing rate decreases. Accordingly, in order to keep enough practical polishing rate to improve manufacturing efficiency, it is important to reduce the clogging in a polishing pad.

SUMMARY OF THE INVENTION

Accordingly, an object of the present invention is to provide a polishing composition which can inhibit a remarkable decrease of a polishing rate caused by the clogging in a polishing pad, and to provide a polishing method using such a polishing composition.

To achieve the foregoing and other objectives and in accordance with the purpose of the present invention, a polishing composition is provided. The polishing composition, used for polishing an object with the use of a polishing pad, includes colloidal silica. The colloidal silica has such a particle diameter as to satisfy the relationship of the inequality: D_(SA)≦D_(N4), between an average primary particle diameter D_(SA) of the colloidal silica calculated on the basis of a BET method and an average secondary particle diameter D_(N4) of the colloidal silica measured by a laser diffraction method, and has the average secondary particle diameter D_(N4) of 30 nm or smaller.

The present invention also provides a polishing method. The method includes preparing the above polishing composition and polishing an object with the use of a polishing pad, by using the prepared polishing composition.

Other aspects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention, together with objects and advantages thereof, may best be understood by reference to the following description of the presently preferred embodiments together with the accompanying drawings in which:

FIG. 1 is a graph showing a relationship between the number of polishing times and a polishing rate.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

An embodiment of the present invention will now be described.

A silicon wafer used for the substrate of a semiconductor device is made of single crystal silicon, and is manufactured through the steps of: cutting out the silicon wafer from an ingot of the single crystal silicon; lapping and etching the surface of the cut silicon wafer; and mirror-finishing the surface of the silicon wafer. In the step of mirror-finishing the surface of the silicon wafer, a polishing pad and a polishing composition are used. The polishing pad is made of a porous material such as a nonwoven fabric, a foam, and a suede cloth. The silicon wafer is polished by contacting the polishing pad with a wafer surface, and relatively sliding the wafer and the polishing pad while supplying the polishing composition to a contacting part.

When clogging occurs in a polishing pad due to particles in a polishing composition, the chippings of the polishing pad, and the cuttings of a wafer, a polishing rate is remarkably decreased. In order to recover the decreased polishing rate, complicated operations such as the dressing or exchange of the polishing pad becomes necessary, which decreases manufacturing efficiency. Then, the present embodiment provides the following polishing composition capable of reducing the clogging in a polishing pad, for the purpose of inhibiting the decrease of the polishing rate caused by the clogging in the polishing pad.

A polishing composition according to the present embodiment contains colloidal silica as an essential component. Colloidal silica in the polishing composition plays a mechanically polishing role for a silicon wafer, which is an object to be polished by the polishing composition. Colloidal silica is a material having a slurry form of dispersing silica particles in a dispersion medium. The dispersion medium is not particularly limited but may be any liquid such as an organic solvent like alcohol, water, and the solution of a surface active agent. However, from the viewpoint of minimizing impurities contained in colloidal silica, water such as ion-exchanged water and distilled water, from which impurities are filtered out by a filter, is preferable.

An average primary particle diameter DSA of colloidal silica is the average particle diameter of the primary particles (single particles) of silica particles in colloidal silica, and is calculated from a specific surface area of the silica particles measured with a BET method, and a particle density of the silica particles. An average secondary particle diameter D_(N4) of colloidal silica is the average particle diameter of secondary particles (agglomerated particles) of the silica particles in colloidal silica, and is measured with a laser diffraction method.

The secondary particles are formed by the agglomeration of the primary particles, so that there necessarily exists the relationship of the inequality: D_(SA)≦D_(N4), between the average primary particle diameter D_(SA) Of colloidal silica and the average secondary particle diameter D_(N4) of colloidal silica. From the viewpoint of avoiding the excessive agglomeration of the primary particles, there preferably exists the relationship of the inequality: D_(N4)≦3×D_(SA), between the average primary particle diameter D_(SA) and the average secondary particle diameter D_(N4) of colloidal silica in the polishing composition.

An average secondary particle diameter D_(N4) of colloidal silica in the polishing composition is essentially 30 nm or smaller, is preferably 25 nm or smaller, and further preferably 20 nm or smaller, from the viewpoint of reducing clogging in a polishing pad. On the other hand, from the viewpoint of securing a practical polishing rate, the average secondary particle diameter D_(N4) is preferably 5 nm or larger. The practical polishing rate means a polishing rate capable of completing polishing for a silicon wafer, in such a period of time as to cause no harm for manufacturing efficiency.

An average primary particle diameter D_(SA) of colloidal silica in the polishing composition is, from the viewpoint of reducing clogging in a polishing pad, preferably 20 nm or smaller, further preferably 15 nm or smaller, and most preferably 10 nm or smaller. On the other hand, from the viewpoint of securing a practical polishing rate, the average primary particle diameter D_(SA) is preferably 3 nm or larger.

The content of silica particles in the polishing composition is, from the viewpoint of reducing clogging in a polishing pad, preferably 50 mass % or less, further preferably 30 mass % or less, and most preferably 20 mass % or less. On the other hand, from the viewpoint of securing a practical polishing rate, the content of silicon particles in the polishing composition is preferably 0.1 mass % or more, further preferably 1 mass % or more, and most preferably 10 masse or more.

Colloidal silica occasionally contains metallic impurities such as iron, nickel, copper, calcium, chromium, zinc, and hydroxides and oxides thereof; and these metallic impurities occasionally deposit on a wafer surface, and diffuse into the wafer in a subsequent heat treatment step to adversely affect the electrical characteristics of the wafer. When preparing an aqueous dispersion of colloidal silica with the use of colloidal silica used in the polishing composition so that the content of colloidal silica in the aqueous dispersion is 20 mass %, from the viewpoint of inhibiting an adverse effect on the electrical characteristics of the silicon wafer, the content of the metallic impurities in the aqueous dispersion is preferably 300 ppm or less, further preferably 100 ppm or less, and most preferably 0.3 ppm or less.

Into a polishing composition according to the present embodiment used in the applications of polishing a silicon wafer, at least either one of an alkaline compound and a chelating agent may be added in addition to colloidal silica. When at least either one of the alkaline compound and the chelating agent is added to the polishing composition, the dispersion medium in colloidal silica functions as a solvent for the alkaline compound or the chelating agent.

An alkaline compound acts as a polishing accelerator for assisting and accelerating mechanical polishing due to colloidal silica through a chemical action such as corrosion and etching. The alkaline compound added into the polishing composition may be an inorganic alkaline compound such as potassium hydroxide, sodium hydroxide, potassium hydrogen carbonate, potassium carbonate, sodium hydrogen carbonate, and sodium carbonate; ammonia; an ammonium salt such as tetramethylammonium hydroxide, ammonium hydrogen carbonate, and ammonium carbonate; or an amine such as anhydrous piperazine, piperazine hexahydrate, 1-(2-aminoethyl)piperazine, N-methylpiperazine, methylamine, dimethylamine, trimethylamine, ethylamine, diethylamine, triethylamine, ethylenediamine, monoethanolamine, N-(β-aminoethyl)ethanolamine, hexamethylenediamine, diethylenetriamine, and triethylenetetramine. Among them, potassium hydroxide, sodium hydroxide, potassium hydrogen carbonate, potassium carbonate, sodium hydrogen carbonate, sodium carbonate, ammonia, tetramethylammonium hydroxide, ammonium hydrogen carbonate, ammonium carbonate, anhydrous piperazine, piperazine hexahydrate, 1-(2-aminoethyl)piperazine, and N-methylpiperazine are preferable because of having a strong action of accelerating the mechanical polishing due to colloidal silica. The number of the type of the alkaline compounds added to the polishing composition may be one or more.

Some alkaline compounds can be chelate-bonded with a metal impurity. However, a bonding force between the alkaline compound and the metal impurity is not so strong that the metal impurity is released from the alkaline compound in the middle of polishing a silicon wafer with the use of the polishing composition, and may contrarily pollute the silicon wafer. For this reason, the alkaline compound added to the polishing composition is preferably the compound that does not form a complex ion with a metal impurity, such as potassium hydroxide, sodium hydroxide, ammonia, and tetramethylammonium hydroxide.

The content of the alkaline compound in a polishing composition is, from the viewpoint of inhibiting the surface of a silicon wafer from being roughened and the polishing composition from gelating due to the excessive addition of the alkaline compound, preferably 10 mass % or less, further preferably 8 mass % or less, and most preferably 5 mass % or less. On the other hand, from the viewpoint of strongly accelerating mechanical polishing due to colloidal silica, the content of the alkaline compound is preferably 0.05 mass % or more, further preferably 0.1 mass % or more, and most preferably 0.5 mass % or more.

A chelating agent forms a complex ion with a metal impurity in the polishing composition to catch it, and inhibits the pollution of a silicon wafer. The chelating agent to be added to the polishing composition may be an acid such as nitrilotriacetic acid, ethylenediamine tetraacetic acid, hydroxy ethylenediamine tetraacetic acid, propanediamine tetraacetic acid, diethylenetriaminepenta-acetic acid, triethylenetetraaminehexa-acetic acid, ethylenediamine tetraethylene phosphonic acid, ethylenediamine tetramethylene phosphonic acid, ethylenediamine tetrakis methylenephosphonic acid, diethylenetriamine pentaethylene phosphonic acid, diethylenetriamine pentamethylene phosphonic acid, triethylenetetramine hexaethylene phosphonic acid, triethylenetetramine hexamethylene phosphonic acid, propanediamine tetraethylene phosphonic acid, and propanediamine tetramethylene phosphonic acid; and may be one of salts with an acid selected among the above acids, such as an ammonium salt, a potassium salt, a sodium salt, and a lithium salt. The number of the type of the chelating agents added to the polishing composition may be one or more.

The content of the chelating agent in a polishing composition is, from the viewpoint of inhibiting the gelation of the polishing composition, preferably 6 mass % or less, further preferably 3 mass % or less, and most preferably 1 mass % or less. On the other hand, from the viewpoint of catching more metal impurities in the polishing composition, the content of the chelating agent is preferably 0.001 mass % or more, further preferably 0.005 mass % or more, and most preferably 0.01 mass % or more.

The present embodiment provides the following advantages.

An average secondary particle diameter D_(N4) of colloidal silica in the polishing composition is set to a value of an average primary particle size D_(SA) of colloidal silica in the polishing composition or larger and 30 nm or smaller. Specifically, both of the primary particle diameter and the secondary particle diameter of colloidal silica in the polishing composition are controlled to as small as 30 nm or smaller. As a result, the polishing composition can reduce clogging in a polishing pad and inhibit remarkable decrease in a polishing rate due to the clogging in the polishing pad.

It should be apparent to those skilled in the art that the present invention may be embodied in many other specific forms without departing from the spirit or scope of the invention. Particularly, it should be understood that the invention may be embodied in the following forms.

To a polishing composition of the above described embodiment, an additive other than an alkaline compound and a chelating agent, such as an antiseptic agent and a surface active agent, may be added. When an anionic surface active agent is added to the polishing composition, clogging in a polishing pad may be greatly reduced, because the anionic surface active agent has an inherent property of improving the dispersibility of negatively charged colloidal silica.

A polishing composition of the above described embodiment may be provided for use after having been diluted. A dilution ratio is, from the viewpoint of securing a practical polishing rate, preferably 50 times or less, further preferably 40 times or less, and most preferably 25 times or less, when the content of silica particles in the polishing composition is 0.1 to 50 mass %.

Instead of the application of polishing a silicon wafer, a polishing composition according to the above described embodiment may be used for the application of polishing a semiconductor device which has a wiring structure formed of copper, aluminum, or an alloy thereof; the application of polishing a substrate for a semiconductor such as silicon-germanium, a substrate for a compound semiconductor such as gallium-arsenic and indium-phosphorus, and a substrate for an oxide such as lithium tantalate, lithium niobate, and sapphire; the application of polishing an aluminum substrate and a glass substrate used in a magnetic recording media for a hard disk drive or the like; and the application of polishing a glass substrate and a resin substrate used in a liquid crystal display and an organic electroluminescent display.

In the next place, Examples and Comparative Examples according to the present invention will be described.

The stock solutions of a polishing composition according to Examples 1 to 16 and Comparative Examples 1 to 7 were prepared by adding each of colloidal silica, an alkaline compound and a chelating agent as needed, into ion-exchanged water. The detail of colloidal silica, an alkaline compound and a chelating agent in each stock solution is shown in Table 1. The polishing compositions according to Examples 1 to 16 and Comparative Examples 1 to 7 were prepared by diluting each stock solution into 20 times with ion-exchanged water. TABLE 1 Alkaline Chelating Colloidal silica compound agent D_(SA) D_(N4) Content Content Content [nm] [nm] [wt %] Kind [wt %] Kind [wt %] Ex. 1 5  9 18 TMAH 2.4 TTHA 0.25 Ex. 2 8  14 18 TMAH 2.4 TTHA 0.25 Ex. 3 9  17 18 TMAH 2.4 TTHA 0.25 Ex. 4 8  19 18 TMAH 2.4 TTHA 0.25 Ex. 5 12  23 18 TMAH 2.4 TTHA 0.25 Ex. 6 14  21 18 TMAH 2.4 TTHA 0.25 Ex. 7 16  19 18 TMAH 2.4 TTHA 0.25 Ex. 8 19  28 18 TMAH 2.4 TTHA 0.25 Ex. 9 9  17 9 TMAH 2.4 TTHA 0.25 Ex. 10 9  17 18 KOH 1.3 TTHA 0.25 Ex. 11 12  23 18 KOH 1.3 TTHA 0.25 Ex. 12 9  17 18 PIZ 5.8 EDTPO 0.24 TMAH 1.0 Ex. 13 12  23 18 PIZ 5.8 EDTPO 0.24 TMAH 1.0 Ex. 14 9  17 18 TMAH 3.6 TTHA 0.25 Ex. 15 9  17 18 TMAH 2.4 — — Ex. 16 9  17 18 TMAH 2.4 DTPA 0.41 C. Ex. 1 22  40 18 TMAH 2.4 TTHA 0.25 C. Ex. 2 10  35 18 TMAH 2.4 TTHA 0.25 C. Ex. 3 35  70 18 TMAH 2.4 TTHA 0.25 C. Ex. 4 90 230 18 TMAH 2.4 TTHA 0.25 C. Ex. 5 16  95 18 TMAH 2.4 TTHA 0.25 C. Ex. 6 35  70 18 KOH 1.3 TTHA 0.25 C. Ex. 7 35  70 18 PIZ 5.8 EDTPO 0.24 TMAH 1.0

The average primary particle diameter of colloidal silica shown in the column entitled “D_(SA)” of Table 1 was calculated on the basis of a specific surface area measured with the use of “Flow Sorb II2300” made by Micromeritics. The average secondary particle diameter of colloidal silica shown in the column entitled “D_(N4)” of Table 1 was measured with the use of “N4 Plus Submicron Particle Sizer” made by Beckman Coulter, Inc. “KOH”, “TMAH”, and “PIZ” in the column entitled “Alkaline compound” of Table 1 respectively indicates potassium hydroxide, tetramethylammonium hydroxide, and anhydrous piperazine. “TTHA”, “EDTPO”, and “DTPA” in the column entitled “Chelating agent” of Table 1 respectively indicates triethylenetetraaminehexa-acetic acid, ethylenediamine tetrakis methylenephosphonic acid, and diethylenetriaminepenta-acetic acid.

After a silicon wafer has been polished with the use of each polishing composition under the following polishing conditions, the thicknesses of the central part of the wafer before and after being polished were measured. Then, a polishing rate was determined by dividing the difference between the thicknesses before and after being polished by a polishing time (15 minutes). The polishing rates of each step after having finished the 5th batch, the 10th batch, and the 15th batch of polishing were determined, and the values are shown in the column entitled “Polishing rate” of Table 2. In addition, the determined values thereof were evaluated based on five scales of A to E. Specifically, the value was evaluated into “A” for a polishing rate of 1.0 μm/minute or more, “B” for a polishing rate of less than 1.0 μm/minute and 0.9 μm/minute or more, “C” for the polishing rate of less than 0.9 μm/minute and 0.8 μm/minute or more, “D” for a polishing rate of 0.8 μm/minute and 0.7 μm/minute or more, and “E” for a polishing rate of less than 0.7 μm/minute and 0.6 μm/minute or more. The evaluation results are also shown in the column entitled “Polishing rate” of Table 2. Here, “X” in the column entitled “Polishing rate” means that the polishing rate extremely decreased before starting the 10th batch or the 15th batch of polishing, and the rest of the polishing was stopped.

The ratio (percentage) of a polishing rate after the 10th batch to that after the 5th batch, and the ratio (percentage) of the polishing rate after the 15th batch to that after the 5th batch were calculated. The results are shown in the column entitled “Maintenance factor of polishing rate” of Table 2. In addition, the calculated maintenance factors of the polishing rates were evaluated based on the four scales of A to D. Specifically, the maintenance factor of 90% or more was evaluated into “A”, the maintenance factor of less than 90% and 80% or more was evaluated into “B”, the maintenance factor of less than 80% and 70% or more was evaluated into “C”, and the maintenance factor of less than 70% and 60% or more was evaluated into “D”. The evaluation results are also shown in the column entitled “Maintenance factor of polishing rate” of Table 2. Here, “X” in the column entitled “Maintenance factor of polishing rate” means that the polishing rate extremely decreased before starting the 10th batch or the 15th batch of polishing, and the rest of the polishing was stopped.

<Polishing Conditions>

-   -   Polishing apparatus: single-sided polishing machine “SPM-15”         provided with four ceramic plates, made by Fujikoshi Machinery         Corp.,     -   Object to be polished: four sheets of 6-inch silicon wafers (of         a p-type, with a crystal orientation of <100> and a resistivity         of 0.1 Ω·cm or more and less than 100 Ω·cm) fixed on each         ceramic plate with an wax,     -   Polishing load: 31.5 kPa,     -   Rotational frequency of press platen: 60 min⁻¹ (60 rpm),     -   Rotational frequency of ceramic plate: 120 min⁻¹ (120 rpm),     -   Polishing pad: Suba800 made by Rodel Corporation without being         dressing from beginning to end     -   Usage of polishing composition: circulating use at the feed rate         of 0.008 m³ (8L) per minute,     -   Polishing hours: 15 minutes per batch, and

Retention temperature for polishing composition: 40° C. TABLE 2 Maintenancc factor of polishing rate [%] Polishing rate [μm/min] 10th batch/ 15th batch/ 5th batch 10th batch 15th batch 5th batch 5th batch Determined Evalu- Determined Determined Determined Determined values ation values Evaluation values Evaluation values Evaluation values Evaluation Ex. 1 1.05 A 0.95 B 0.90 B 90 A 86 B Ex. 2 1.08 A 1.00 A 0.91 B 93 A 84 B Ex. 3 1.07 A 1.01 A 0.91 B 94 A 85 B Ex. 4 1.11 A 1.00 A 0.90 B 90 A 81 B Ex. 5 1.05 A 0.95 B 0.87 C 90 A 83 B Ex. 6 1.04 A 0.96 B 0.86 C 92 A 83 B Ex. 7 1.04 A 0.94 B 0.79 D 90 A 76 C Ex. 8 1.04 A 0.93 B 0.78 D 89 B 75 C Ex. 9 1.02 A 0.88 C 0.75 D 86 B 74 C Ex. 10 0.81 C 0.81 C 0.81 C 100 A 100 A Ex. 11 0.81 C 0.80 C 0.80 C 99 A 99 A Ex. 12 1.02 A 0.81 C — X 79 C — X Ex. 13 1.01 A 0.80 C — X 79 C — X Ex. 14 1.10 A 1.01 A 0.92 B 92 A 84 B Ex. 15 1.07 A 1.01 A 0.90 B 94 A 84 B Ex. 16 1.06 A 1.01 A 0.91 B 95 A 86 B C. Ex. 1 1.06 A 0.82 C 0.73 D 77 C 69 D C. Ex. 2 1.05 A 0.80 C 0.72 D 76 C 69 D C. Ex. 3 1.08 A 0.83 C 0.74 D 77 C 69 D C. Ex. 4 1.03 A 0.72 D — X 70 C — X C. Ex. 5 1.01 A 0.62 E — X 61 D — X C. Ex. 6 0.82 C 0.80 C 0.74 D 98 A 90 A C. Ex. 7 1.00 A — X — X — X — X

As shown in Table 2, Examples 1 to 8 provided very excellent evaluation results on both a polishing rate and the maintenance factor of the polishing rate, in comparison with Comparative Examples 1 to 5. From the results, it was known that the polishing compositions in Examples 1 to 8 hardly causes clogging in a polishing pad and can inhibit the polishing rate from decreasing due to the clogging in the polishing pad. In addition, from the results on the maintenance factors of the polishing rate in Example 8 and Comparative Example 2, it was known that by an average secondary particle diameter D_(N4) of colloidal silica set to 30 nm or smaller further inhibits the polishing rate from decreasing due to repeated polishing. In addition, from the results of the maintenance factor of the polishing rate in Example 8 and Comparative Example 1, it was known that an average primary particle diameter D_(SA) of colloidal silica set to 20 nm or smaller also further inhibits the polishing rate from decreasing. Example 9 contained a slightly small amount of colloidal silica in the polishing composition to show the result of the slightly decreasing polishing rate at the 15th batch. Examples 10 and 11 provided very excellent evaluation results on both the polishing rate and the maintenance factor of the polishing rate in comparison with Comparative Example 6. Examples 12 and 13 provided superior evaluation results on both the polishing rate and the maintenance factor of the polishing rate in comparison with Comparative Example 7. From the results of Examples 3 and 10 and Examples 5 and 11, it was known the use of potassium hydroxide for an alkaline compound further improves the maintenance factor of the polishing rate. From the results of Examples 3 and 14 to 16, it was known that the content of the alkaline compound in the polishing composition and the presence or absence of the addition of a chelating agent does not significantly affect the polishing rate and the maintenance factor of the polishing rate.

A relationship between the number of polishing times (the number of batches) and a polishing rate, is shown on Examples 3 and 8 and Comparative Example 3 in a graph of FIG. 1. As shown in FIG. 1, the number of polishing times before the polishing rate drops below 0.60 μm/minute increases, from the lowest to the highest, in Comparative Example 3, Example 8, and Example 3. From the result, it was known that the smaller is an average secondary particle diameter D_(N4) of colloidal silica, the more hardly occurs clogging in a polishing pad, and the more effectively inhibited is the polishing rate from decreasing due to the clogging in the polishing pad.

The present examples and embodiments are to be considered as illustrative and not restrictive and the invention is not to be limited to the details given herein, but may be modified within the scope and equivalence of the appended claims. 

1. A polishing composition used for polishing an object with the use of a polishing pad, the polishing composition comprising colloidal silica, wherein the colloidal silica has such a particle diameter as to satisfy the relationship of the inequality: D_(SA)≦D_(N4), between an average primary particle diameter D_(SA) of the colloidal silica calculated on the basis of a BET method and an average secondary particle diameter D_(N4) of the colloidal silica measured by a laser diffraction method, and has the average secondary particle diameter D_(N4) of 30 nm or smaller.
 2. The polishing composition according to claim 1, wherein the colloidal silica has the average secondary particle diameter D_(N4) of 20 nm or smaller.
 3. The polishing composition according to claim 1, wherein the colloidal silica has the average secondary particle diameter D_(N4) of 5 nm or larger.
 4. The polishing composition according to claim 1, wherein the colloidal silica has the average primary particle diameter D_(SA) of 20 nm or smaller.
 5. The polishing composition according to claim 4, wherein the colloidal silica has the average primary particle diameter D_(SA) of 10 nm or smaller.
 6. The polishing composition according to claim 1, wherein the colloidal silica has the average primary particle diameter D_(SA) of 3 nm or larger.
 7. The polishing composition according to claim 1, wherein the colloidal silica has such a relationship as to satisfy the inequality: D_(N4)≦3×D_(SA), between the average primary particle diameter D_(SA) and the average secondary particle diameter D_(N4).
 8. The polishing composition according to claim 1, further comprising an alkaline compound.
 9. The polishing composition according to claim 1, further comprising a chelating agent.
 10. The polishing composition according to claim 1, further comprising an anionic surface active agent.
 11. A polishing method comprising: preparing a polishing composition including colloidal silica which has the particle diameter of satisfying the relationship of the inequality: D_(SA)≦D_(N4), between an average primary particle diameter D_(SA) of the colloidal silica calculated on the basis of a BET method and an average secondary particle diameter D_(N4) of the colloidal silica measured by a laser diffraction method, and has the average secondary particle diameter D_(N4) of 30 nm or smaller; and polishing an object with the use of a polishing pad, by using the prepared polishing composition. 